The natural ligaments are elongated bundles of collagenous soft tissue that serve, among other things, to hold the component bones of joints together. The surgical treatment of diseased or damaged ligaments, e.g. the anterior cruciate ligament, has been severely hampered by the unavailability of a suitable, generally accepted ligament prosthesis. The desired characteristics for a ligament prosthesis include appropriate size and shape, biological compatibility, capability of being readily attached by the surgeon to the body of the patient, high fatigue resistance and mechanical behavior approximating that of the ligamentous tissue sought to be repaired or replaced.
The latter desired characteristic is particularly important. Natural ligaments are both strong and highly elastic, which qualities are generally not found together in a single material. Thus, for example, the anterior cruciate ligament of normal adult humans exhibits a yield point in tension of about 50 kg. at a reversible elongation of about 28%, and a break point of about 60 kg. (Typical adult human tendons are stronger and less elastic.) A number of ligament and/or tendon prostheses are known in which the load bearing body portion is fabricated essentially of a single synthetic material (see, e.g., U.S. Pat. Nos. 3,176,316; 3,613,120; 4,127,902; 4,149,277; 4,209,859; 4,255,820; 4,329,743 and 4,345,339; U.K. Pat. No. 1,602,834 and European Published Patent Appln. 51,954). These monocomponent devices generally possess insufficient longitudinal elasticity and some also exhibit inadequate longitudinal break strength. As a result of their insufficient elasticity, this type of prosthesis must be forced into the region of plastic deformation to achieve the longitudinal elongation desired for normal anatomical function, e.g. flexion of a joint, which of course permanently impairs the mechanical function of the prosthesis.
Recently, ligament prostheses have been disclosed in U.S. Pat. Nos. 4,246,660 and 4,301,551 in which the load bearing body portion is a bicomponent structure comprised of one material that imparts strength to the prosthesis and another material that imparts elasticity. The use of these prostheses alleviates the disadvantages described above for the monocomponent type of prosthesis. However, the prostheses disclosed in the '660 and '551 Patents are complex in construction and their methods of attachment to the body of the patient involve rather complicated surgical procedures.
A recent thesis (Elizabeth E. Fitzgerald, "Mechanical Behavior of Bicomponent Braids as Potential Surgical Implants", Master of Science Thesis, Cornell University, August 1979) has disclosed the use of a braided bicomponent tube as a ligament prosthesis. In this prosthesis two interwoven sets of polymeric fibers, one of a strong material and the other of an elastic material, are helically-disposed in the wall of the tube and oriented at a fixed angle with respect to one another. Each set of fibers is oriented at the same acute angle with respect to the longitudinal direction of the tube. The prosthesis may additionally comprise a monocomponent polymeric filament core.
The prosthesis disclosed in the Fitzgerald thesis has certain inherent disadvantages. First, since the fibers in the two helically-disposed interwoven sets are not idential, the prosthesis is not balanced and will tend to twist during longitudinal elongation. Second, since the set of helically-disposed elastic fibers is angulated with respect to the longitudinal direction of the prosthesis, only a minor amount of the work performed in elongating the prosthesis longitudinally is converted to elastic energy stored in the extended set of elastic fibers. Undesirably large portions of said work are converted to elastic energy stored in the other set of strong fibers or dissipated as friction in the extending trellis-like bicomponent braided structure.